One of President Vladimir Putin's closest allies said Russia's military operation in Ukraine had not all gone as quickly as the Kremlin had wanted, the strongest public acknowledgment yet from Moscow that things were not going to plan.

National Guard chief Viktor Zolotov, speaking at a church service led by Orthodox Patriarch Kirill on Sunday, blamed the slower than expected progress on what he said were far-right Ukrainian forces hiding behind civilians, an accusation repeatedly made by officials in Russia.

His comments appeared at odds with an assessment on Friday by Russia's Defence Minister Sergei Shoigu who told Putin that "everything is going according to plan."

"I would like to say that yes, not everything is going as fast as we would like," Zolotov, once in charge of Putin's personal security, said in comments posted on the National Guard's website.

"..But we are going towards our goal step by step and victory will be for us, and this icon will protect the Russian army and accelerate our victory."

The United States and its European allies have cast Putin's invasion as an imperial-style land grab that has so far been poorly executed because Moscow underestimated Ukrainian resistance and Western resolve to punish Russia.

Zolotov is a powerful security official who was once Putin's security chief.

He now heads the National Guard, a kind of internal military force that includes the riot police and other forces. It is subordinated directly to Putin and has forces deployed in Ukraine.